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The Rubric of Ahriman was the name given to an immensely powerful ritual spell cast by a cabal of Chaos Sorcerers from amongst the Thousand SonsTraitor Legion, who were led by the XV Legion's former Chief LibrarianAhzek Ahriman, not long after the Legion had fled from their homeworld of Prospero to the Planet of the Sorcerers during the Horus Heresy. When mutation started to run rampant once again through the ranks of the Thousand Sons due to their exposure to the Warp energies of the Eye of Terror, Ahriman delved deep into the pages of the Book of Magnus, a massive tome compiled by the Legion's PrimarchMagnus the Red, that was filled with forbidden lore and sorcerous knowledge from ancient, forgotten days. Ahriman believed the Book of Magnus held the key to his Legion's salvation. In the labyrinthine collections of formulae, incantations, and rites, Ahriman devised what he believed would be the beginnings of a mighty arcane spell to undo all that had befallen his Battle-Brothers. Unfortunately, Ahriman miscalculated, for while the spell did stop the rampant mutation, it did so by transforming the Chaos Space Marines of the Legion who lacked psychic abilities into mindless, undead spectres known as Rubric Marines, whose souls are trapped forever within their ancient suits of Power Armour.

The Rubric of Ahriman was a cataclysmic end to the existence of the Thousand SonsLegion within the Imperium of Man. When the Emperor of Mankind neared completion of the Primarch Project in the late 30th Millennium, he enhanced the psyker genes that existed within one of the nascent Primarch zygotes he had genetically engineered. The being that would be known as Magnus the Red became self-aware during his gestation, and even managed to have some limited psychic contact with the Emperor before the 20 Primarchs were spirited away and scattered across the galaxy through the Warp by the Ruinous Powers. The Emperor was greatly pleased by this, as he thought that at least one of his sons would be a potent psyker and would be able to share some of the burden he bore as humanity's chief psychic protector. Alas, Magnus' genome was ultimately flawed, and when the Emperor used what remained of it to create the gene-seed of the XV Space Marine Legion, this flaw manifested itself in what was then called the "flesh-change," a mutation that would strike down Space Marine after Space Marine and devolve them from superhuman warriors to a ravening masses of swiftly mutating flesh. This often led to them becoming what later generations would recognise as Chaos Spawn who needed to be purged by flame.

Amongst those recruited to form the core of the XV Legion was a young warrior and psyker known as Ahzek Ahriman. Like many of his brethren, he futilely searched for a cure to the "flesh-change". He was forced to put down his twin brother, Ohrmuzd, after seeing him succumb to the change, and was on the verge of falling to it himself until the re-discovery of the Thousand Sons' Primarch had brought them salvation from some unknown means and miraculously cured the XV Legion of this affliction. But unknown to his sons, Magnus had been tricked by Tzeentch, striking a bargain with the Changer of Ways, who supposedly gave the Primarch a "cure" for the Thousand Sons' affliction. In his arrogance, Magnus believed that this curse was behind his Legion, for he had ended the degradation of the XV Legion's gene-seed and restored biological harmony to the Thousand Sons. But this supposed "cure" proved only a temporary fix for the rampant "flesh-change." United in their suffering, the Battle-Brothers of the XV Legion, now known as Magnus' Thousand Sons, formed a close-knit brotherhood, fanatically devoted to their Primarch and saviour. Ahriman idealised his Primarch like others in his Legion and sought to emulate him in all things. He eventually rose to the position of Chief Librarian; second-in-command and Magnus' closest advisor. Such was their bond that Magnus entrusted Ahriman with the keeping of the Book of Magnus.

The first chink in Ahriman's faith in Magnus appeared on the world of Shrike in the Ark Reach Cluster where, unexpectedly, the "flesh-change" struck again, mutating one Thousand Son Astartes in full view of the Word Bearers and the Space Wolves. Magnus shrugged the incident off, believing that the affected Thousand Son was merely an aberrant mutation, a one in a billion fluke. The Primarch believed that one unique failure of his "cure" was still a more than acceptable result. But the first doubts that his Primarch was not the almighty, all-knowing being he believed him to be crept into Ahriman's mind. These doubts would only fester and grow during the events surrounding the Burning of Prospero, and by the time the Thousand Sons had been "rescued" by Tzeentch and Magnus had become a Daemon Prince of the Changer of Ways, Ahriman's love and admiration had turned to hatred and contempt. When the "flesh-change" once again ran rampant amongst the survivors of the Thousand Sons and Magnus seemed to accept it, Ahriman set out to find a cure for the "flesh-change" himself.

Consumed by his own hatred and hubris and utterly unaware of the ludicrousness of attempting to use the very energy of change to stop change, Ahriman delved into the sorcerous knowledge held in the Book of Magnus. Distilling the colossal collections of formulae, incantations and rites, and infusing the results with his own hatred of Magnus and angst at the fate of his Legion, Ahriman devised the canvas of a mighty arcane spell that would ultimately undo all the woe that had befallen his Battle-Brothers and thus protect them from the "flesh-change" for all eternity. Preliminary testing of the spell he called the Rubric generated great promise,but he quickly discovered that he lacked the raw power required to achieve permanent results. He then set to work gathering those amongst the remaining officers of the Thousand Sons who, like him, were disgusted by what had befallen their Legion and their Primarch. Gathering in a fell circle around Ahriman, this cabal of Sorcerers lent their power to Ahriman, who then unleashed the full potential of his desperate spell.

The Rubric manifested itself as an immense storm of arcane power that engulfed the Planet of the Sorcerers in an etheric hurricane of multi-coloured clouds. Daemons who witnessed the phenomenon fled into the deep Warp for their very existences, as the abomination unleashed by Ahriman could snuff out even their immortal essence. Once fully empowered, the maelstrom of energies fell upon the Thousand Sons as bolts of lightning. Those amongst them that were psykers absorbed the lightning and had their psychic powers drastically augmented. However, the majority of Battle-Brothers of the Legion who lacked the psychic gift could not deal with the cataclysmic amounts of sorcerous energy which poured into them. Their flesh burned on the spot, their bodies reduced to ash inside their armour. And yet, the energies released sealed all the joints of their Power Armour as it burned their bodies. When their souls attempted to depart their ruined bodies, they found themselves trapped inside their armour; dead, yet still alive, without a body but unchanging for all eternity.

To those Thousand Sons that survived the attentions of the spell, the ordeal was far from over. They could feel their brethren being utterly consumed through the psychic link that united them. The most affected by this was Magnus himself, who felt every bolt, every burn and every entrapment as if it were on his own armour, body and soul. Disabled by the overwhelming sensations, it was only after his whole Legion had been affected that the Primarch intervened, wresting control of the spell away from Ahriman and putting an end to it when his Chief Librarian could not.

In the aftermath of the cataclysm, it became clear that the Rubric had either succeeded beyond all possible expectations or failed abominably, depending on how one looked at it. Instead of purging the flesh of ravening mutations, each of the Thousand Sons had been transformed. The seals and joints of their Power Armour had been welded shut, and the body within turned to ashes. What remained was a suit of animated armour, devoid of mutation, but of all sentience and will as well. Ahriman had created what would later be known as the Rubric Marines. Ahriman himself, along with most of his cabal, was ecstatic: his brethren were now as he intended, protected from the "flesh-change," and the fact that they paid for this protection with the destruction of their physical bodies was in his eyes an acceptable price to pay. Others, including Magnus and Amon, were horrified that their once-proud fellow Astartes were now little more than spectral automatons, barely aware of the world around them, what remained of their minds irrevocably shattered by the ordeal. (It is to be noted that Ahriman's opinion has evolved over time, and that today he considers the Rubric as a painful but necessary evil rather than the miracle solution he once believed it to be.)

Magnus, angered beyond reason, assaulted the tower where Ahriman had gathered his coven. The other Chaos Sorcerers immediately knelt and abased themselves before Magnus' fury, but Ahriman remained standing, vindicated in his beliefs and utterly unrepentant. As Magnus' psychic might seized him and forced him down, he managed to hiss one spiteful sentence: "I succeeded where you failed, Magnus...," hatefully stabbing and twisting the blade of guilt into his Primarch's heart as deep and as hard as he could. Enraged, Magnus raised his immense fist to strike down Ahriman, but another voice softly insinuated itself into Magnus' mind, gently laughing and whispering, "Magnus, would you smash my pawns so readily?." For unknown reasons, Tzeentch himself interceded, staying the Daemon Prince's hand. Why he did so remains unknown. Some say that even the Architect of Fate was impressed by the Rubric, yet others whisper that the whole incident was nothing but another plot masterminded by the Great Conspirator and played out by his unwitting pawns.

To avoid disrupting his master's future plans for Ahriman, Magnus sent the Sorcerer on a quest to understand the nature of Tzeentch and the Warp, banishing him and his cabal from the Planet of the Sorcerers until the day their quest had been fulfilled. Since that time over 10,000 Terran years ago, Ahriman and his warband of Thousand Sons Chaos Space Marines, the Prodigal Sons, have wandered across the galaxy in their quest to gain a better understanding of the nature and power of Chaos. The remaining Thousand Sons, now reduced mostly to spectral Rubric Marines, have marched into battle led by apprentice Chaos Sorcerers, who are the only members of the Thousand Sons with the ability to command the mindless beings that now lurk within the armour of the once-proud XV Legion.